Multiply.
When a number is next to a variable, it means they are multiplied. So, $({-4}{r^3})({-5}{r^3})$ is the same as $({-4})({r^3})({-5})({r^3})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(-4)(-5)}{(r^3)(r^3)}$ Then simplify, and we're done! ${20}{r^{6}}$